1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an X-ray source, an X-ray detector and an X-ray filter which is arranged between the X-ray source and the X-ray detector and includes a plurality of filter elements having an X-ray absorptivity which can be adjusted by controlling a quantity of X-ray absorbing liquid within the individual filter elements. The invention also relates to a method of setting an X-ray examination apparatus, involving the adjustment of the X-ray absorptivity of filter elements of an X-ray filter by controlling a quantity of X-ray absorbing liquid within the individual filter elements.
2. Description of the Related Art
An X-ray examination apparatus and a method of this kind are known from French Patent Application FR 2 599 886.
The known X-ray examination apparatus comprises a filter for limiting the dynamic range of an X-ray image, being the interval between the extremes of the brightness values. An X-ray image is formed on the X-ray detector by arranging an object, for example a patient to be examined, between the X-ray source and the X-ray detector and by irradiating said object by means of X-rays emitted by the X-ray source. If no steps are taken, the dynamic range of the X-ray image may be large. On the one hand, for some parts of the object, for example lung tissue, the X-ray transmittance is high whereas other parts of the object, for example bone tissue, can hardly be penetrated by X-rays. If no further steps are taken, therefore, an X-ray image is obtained with a large dynamic range whereas, for example, medically relevant information in the X-ray image is contained in brightness variations in a much smaller dynamic range; because it is not very possible to make small details of low contrast nimbly visible in a rendition of such an X-ray image, the image is not very well suitable for making a diagnosis. If, using an image-intensifier pick-up chain, the X-ray image is converted into an optical image which is picked up by means of video camera, the dynamic range of the optical image could be larger than the range of brightness values that can be handled by the video camera without causing disturbances in the electronic image signal.
In order to limit the dynamic range of the X-ray image the known X-ray examination apparatus comprises a filter with filter elements provided with a bundle of parallel capillary tubes, each of which is connected, via a valve, to a reservoir containing an X-ray absorbing liquid which suitably wets the inner walls of the capillary tubes. In order to fill a capillary tube with the X-ray absorbing liquid, the valve of the relevant capillary tube is opened, after which the capillary tube is filled with the X-ray absorbing liquid by the capillary effect. Such a filled capillary tube has a high absorptivity for X-rays passing through such a filled capillary tube in a direction approximately parallel to its longitudinal direction. The valves are controlled so as to ensure that the amount of X-ray absorbing liquid in the capillary tubes is adjusted in such a manner that in parts of the X-ray beam which pass through object parts of low absorptivity filter elements are adjusted to a high X-ray absorptivity and that filter elements in parts of the X-ray beam which pass through object parts of high absorptivity or are intercepted by a lead shutter are adjusted to a low X-ray absorptivity.
In order to change the setting of the filter of the known X-ray examination apparatus it is necessary to empty filled capillary tubes first. Therefore, use is made of a paramagnetic X-ray absorbing liquid which is removed from the capillary tubes by application of a magnetic field. After all capillary tubes have been emptied, the filter is adjusted anew by de-activation of the magnetic field and by subsequently opening valves of capillary tubes which are filled with the X-ray absorbing liquid so as to adjust these tubes to a high X-ray absorptivity in the new filter setting. Consequently, it is not very possible to change the setting of the known filter within a brief period of time, for example one second. Therefore, the known X-ray apparatus is not suitable for the formation of successive X-ray images at a high image rate where the setting of the filter is changed between the formation of successive X-ray images.
Control of the quantity of X-ray absorbing liquid in the capillary tubes necessitates accurate control of the period of time during which the valves are open; however, because the mechanical driving of the valves involves, for example inertia and play, fast and accurate control of the quantity of X-ray absorbing liquid in the capillary tubes is not very well possible.
An object of the invention is to provide an X-ray examination apparatus which comprises an X-ray filter which can be adjusted more quickly and more accurately than the known filter.
To this end, an X-ray examination apparatus in accordance with the invention is characterized in that it comprises an adjusting unit for applying an electric voltage to the individual filter elements, which adjusting unit comprises a timer unit for controlling the period of time during which the electric voltage is applied to the filter elements.
The relative quantity of liquid is to be understood to mean herein the quantity of liquid in such a filter element compared to the quantity of liquid in the relevant filter element when it is completely filled with the liquid. The electric voltage applied to a filter element influences the adhesion of the X-ray absorbing liquid to the inner side of the relevant filter element and this adhesion determines the degree of filling of the filter element with the X-ray absorbing liquid. The relative quantity of X-ray absorbing liquid in individual filter elements is controlled on the basis of the electric voltages applied to individual filter elements. As the electric voltage is applied to such a filter element for a longer period of time, the relative quantity of X-ray absorbing liquid in the relevant filter element increases and hence the X-ray absorptivity of said filter element also increases. Depending on the period of time during which the electric voltage is applied, electric current is applied to a filter element which is thus electrically charged. The relative quantity of liquid in the relevant filter element, and hence the X-ray absorptivity, is dependent on the electric charge on the relevant filter element. Because the period of time during which the electric voltage is applied to the individual filter elements can be accurately controlled, the relative quantity of X-ray absorbing liquid can be accurately controlled and hence also the X-ray absorptivity of the individual filter elements. In order to change the setting of the X-ray absorptivity of the filter elements it is not necessary to empty the filter elements first, so that changing the setting of the filter requires a short time only, such as one or a few seconds.
A preferred embodiment of an X-ray examination apparatus in accordance with the invention is characterized in that the timer unit is arranged to apply the electric voltage to individual groups of filter elements during a continuous period of said controllable duration.
As soon as the electric voltage is applied to a filter element, the X-ray absorbing liquid adheres to the inner side of said filter element so that the latter is fired with the X-ray absorbing liquid; filling continues, for as long as the electric voltage is applied, until, if desired, the filter element has been completely filled. As soon as the electric voltage is switched off, the adhesion no longer increases so that the filter element is not filled further. The filter setting is realized by a simple switching procedure by applying the electric voltage to individual groups of filter elements for a continuous period of time of desired duration. If differences are required between the X-ray absorptivities of individual, single filter elements, such a group of filter elements may also comprise a single filter element. Another simple switching procedure concerns the application of the electric voltage to groups of filter elements within a continuous period of time in which the electric voltages are applied to individual filter elements wig such a group during periods of time of different lengths. In an X-ray filter comprising a matrix of filter elements such a group is formed, for example by a row or column of filter elements. In this example filter elements are driven per row or per column within individual, continuous periods.
A further preferred embodiment of an X-ray examination apparatus in accordance with the invention is characterized in that the timer unit is arranged to apply the electric voltage alternately to individual groups of filter elements, repeatedly during separate sub-periods.
The flowing of X-ray absorbing liquid into the filter elements requires electric work which is supplied by the electric charging of a capacitor formed by the filter element whose capacitance varies as a function of the relative quantity of X-ray absorbing liquid in the relevant filter element. Because of the inertia of the flowing in of the X-ray absorbing liquid, the electric work cannot be performed within an arbitrarily short period of time. By delivering the charge to groups of individual filter elements in a number of time discrete fractions, individual groups, for example rows or columns, are at least partly simultaneously filled with the X-ray absorbing liquid. Because individual groups are filled with X-ray absorbing liquid in parallel instead of serially, individual filter elements are effectively given more time so as to be filled with the X-ray absorbing liquid, but the total adjusting time of the filter is not prolonged. According to this method of setting the filter, the filter elements are more or less simultaneously adjusted so that the rendition of the X-ray image can be suitably used for diagnostic purposes also during the setting of the filter.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.